In transporting charged-particle beams it is often necessary to "steer" or guide the beam through, for instance, beam-defining apertures using magnetic deflection yokes or electric deflectors. In most charged particle systems, there is an alignment scheme consisting of either a single (or multiple) electric or magnetic deflectors as described below. This disclosure applies to either electric or magnetic deflectors.
The path that a charged-beam particle follows through a deflector is governed by the Lorentz equation F=q (E+V/c.times.B), where E and B are the electric and magnetic fields, V is the particle's velocity, and q is the charge of the particle. For positively-charged particles, the force is in the same direction as the electric field, and in the case of a magnetic field, it is in a direction mutually perpendicular to the particle's velocity and the magnetic field. The particle's trajectory, governed by these forces, is parabolic in the field. Outside of the field, the trajectory follows a tangential path.
If one follows the trajectory of a charged-particle beam traveling through a single uniform deflector and connects the tangents of the trajectory before and after the deflection, they meet on the axis in the symmetry plane along the axis of the deflector. The beam bundle appears to pivot or be deflected from this point.
A pair of deflectors can be used to deflect a charged particle beam such that the beam appears to be deflected from any point controlled by the relative strengths of the two deflectors, referred to as the deflection center. For instance, in the case of two identical deflectors connected in series, the beam appears to be deflected from the center of the deflector pair. If they are connected in the opposite sense, the beam appears to be deflected from infinity. By varying the relative strength of the two deflectors, the deflection can be made to appear to come from anywhere above or below the deflector pair as shown in T. V. Landon et al., IBM Technical Disclosure Bulletin, July 1976, Vol. 19., No. 2, 464.
In the past, one pair of deflectors was used to generate each deflection center positioned arbitrarily along the axis.